(87d) Fouling in Depropanizer System Despite Inhibitor Treatment - Why Pipe Isometry Is Important

Severe fouling was observed in a gas cracker depropanizer system due to the inability to treat stagnant volumes with inhibitor. This cracker has one active and one spare depropanizer tower. Inhibitor treatment and residuals monitoring for the light ends were active and effective in maintaining operations of the active tower and reboilers. Over time, however, the frequency of reboiler swaps increased significantly. During an opportunity to inspect the piping at the tower outlet, the team discovered that an entire section of 18” ID pipe was full of polymer (mainly polybutadiene). It was determined that this was because this volume was stagnant. Without proper circulation, the inhibitor treatment was not able to slow down fouling and polymer started to build up from the blinded face of the pipe back into the tee where the active flow circulates. Residuals were unable to detect the symptoms of fouling in this section of pipe because the active flow was not interacting much with the radicals present in the growing polymer. Two main learnings came from this event and the investigation. First, one must evaluate carefully the isometry of the piping and ensure that all possible stagnant zones are either treated, recirculated or eliminated. In the case of this cracker, a project was implemented to have a side stream of active (treated) flow circulate back into the one end of the stagnant volume. Second, regular monitoring of potential stagnant areas by x-rays was recommended to prevent catastrophic events due to pipe rupture. This paper describes the sequence of events leading to the main incident, the diagnostic and analytical used to evaluate the risk of the problem and the solution implemented. This serves as a great example to the Olefins industry that just treatment with antifouling is not sufficient unless complemented with an audit of all possible stagnant volumes.